The recent growth in the demand for broadband services has resulted in a pressing need for increased capacity on existing communication channels. The increased bandwidth of fibre optic communication fibres is still often insufficient to cope with this demand without utilising the ability of these fibres to carry large numbers of individual communication channels each identified by the particular wavelength of the light. This technique is known as dense wavelength division multiplexing (DWDM). The disadvantage of this technique is that the increasing density of wavelength channels places increasing demand on network functionality for connecting the individual channels to individual destination points on a dynamic basis, and for the ability to add or drop an individual wavelength channel into or out of the optical signal. Currently these functions are primarily performed by electronic techniques but the demand for increased network speed calls for these functions to be performed in the optical domain.
The use of wavelength selective switching for applications of optical cross-connects has attracted much interest because of the goal of fully flexible networks, where the paths of each wavelength can be reconfigured to allow for arbitrary connection between nodes with the capacity appropriate for that link at a particular point in time. Although this goal is still valid, it is clear that optical networks will evolve to this level of sophistication in a number of stages. The first stage of the evolution is likely to be that of a reconfigurable add/drop node where a number of channels can be dropped or and added from the main path, whose number and wavelength can be varied over time—either as the network evolves or dynamically as the traffic demands vary.
A further functionality demanded by optical communications networks is the ability to route incoming signals from two origins in the same fashion independently of each other in a single device. This immediately halves the device count required at any particular location, without the loss of functionality in the adding and dropping of channels from either source.
This present invention is directed to applications such as dual-source reconfigurable optical add/drop multiplexer (ROADM) networks, dual-source wavelength reconfigurable cross-connects referred to generically as Wavelength Selective Switches (WSS), dual-source dynamic channel equalisation (DCE) and for single-source devices for correction of polarisation-dependant loss (PDL) mechanisms.
The characteristics of a wavelength selective element which is ideal for the applications of Optical Add/Drop and Wavelength Selective Switching can be summarized follows:                i) scalable to multiple fibre ports;        ii) one channel per port or multiple channels per port operation;        iii) reconfiguration of wavelength selectivity to different grids e.g. 50 GHz or 100 GHz or a combination of both;        iv) low optical impairment of the express path;        v) low losses on the drop and express paths;        vi) ability to add and drop wavelengths simultaneously;        vii) ability to be reconfigured between any ports or between any wavelengths without causing transient impairments to the other ports;        viii) equalisation of optical power levels on express path (OADM) or all paths (WSS);        ix) provision of shared optical power between ports for a given wavelength (broadcast mode);        x) flat optical passband to prevent spectral narrowing;        xi) power off configurations that leave the express path of an OADM undisturbed; and        xii) small power and voltage and size requirements.        
In reviewing the many technologies that have been applied it is necessary to generalize somewhat, but the following observations can be made.
Two basic approaches have been made for the OADM and WSS applications.                i) The first has been based on wavelength blocking elements combined with a broadcast and select architecture. This is an optical power intensive architecture, which can provide for channel equalization and reconfiguration of wavelength selectivity, but is not scalable to multiple ports, has very high loss and because of the many auxiliary components such as wavelength tuneable filters has a large power and footprint requirement.        ii) Wavelength switches have been proposed for OADMs, but do not naturally provide for channel equalization, the channel by channel switching in general leads to dispersion and loss narrowing of optical channels, and in the case of multiple port switches it is generally not possible to switch between ports without causing impairment (a hit) on intermediate ports. In addition the channel spacing cannot be dynamically reconfigured. Tuneable 3-port filters have also been proposed having a lack of impairment to the express paths but do not scale easily to multiple ports and may suffer from transient wavelength hits during tuning. Tuneable components are usually locked to a particular bandwidth which cannot be varied. In addition poor isolation of tunable 3 ports means they are less applicable to many add/drop applications which demand high through path isolation.        
One technology that has been applied to optical cross connects has become known as 3-D MEMS utilises small mirror structures which act on a beam of light to direct it from one port to another. Examples of this art are provided in U.S. Pat. Nos. 5,960,133 and 6,501,877. The ports are usually arranged in a 2-dimensional matrix and a corresponding element of the 2-dimensional array of mirrors can tilt in two axes to couple between any one of the ports. Usually two arrays of these mirrors are required to couple the light efficiently and because of the high degree of analogue control required structures based on this technology have proved to be extremely difficult to realize in practice and there are few examples of commercially successful offerings. In this type of structure, a separate component is required to separate each wavelength division multiplexed (WDM) input fibre to corresponding single channel/single fibre inputs.
One of the most promising platforms for wavelength routing application relies on the principle of dispersing the channels spatially and operating on the different wavelengths, either with a switching element or attenuation element. These technologies are advantageous in that the switching element is integrated with the wavelength dispersive element—greatly simplifying the implementation. The trade-off is that in general the switching is more limited, with most implementations demonstrated to date being limited to small port counts—and the routing between ports is not arbitrary. In general a diffraction grating is used for micro-optic implementations or an array waveguide grating for waveguide applications. Most of the switching applications have been based on MEMS micro mirrors fabricated in silicon and based on a tilt actuation in one dimension. The difficulty with this approach has been that to achieve the wavelength resolution required when the angular dispersion is mapped to a displacement. In such cases, an image of the fibre (with or without magnification) is mapped onto the tilt mirror array. In order to couple the light into a second port, additional optical elements are required that convert the angle into a displacement. Different approaches to this have included retroreflection cubes wedges (U.S. Pat. No. 6,097,519) which provide discrete displacements or Angle to Displacement elements (U.S. Pat. No. 6,560,000) which can provide continuous mapping using optical power provisioned at the Rayleigh length of the image. In all of these cases, in order to switch between ports, the tilt mirror needs to pass through the angles corresponding to intermediate ports. In addition, the number of ports is limited in each of these cases by the numerical aperture of the fibre as each of the different switch positions are discriminated by angles. For a fibre with a numerical aperture of 0.1, a switch which can tilt by +−12 degrees could not distinguish 8 different switch positions. One approach that can be used is to decrease the numerical aperture through the use of thermally expanded cores or micro lenses—but this is done at the expense of wavelength resolution.
An alternative has been to use polarization to switch between ports. Obviously this is most appropriate to switching between 2 ports corresponding to the 2 polarisation states. Such a switch is described in Patel (J. S. Patel and Y. Silverberg, IEEE Photonics Technology Letters Vol. 7 No. 5, 1995, pp. 514-516) where an optical dispersion element (in this case a grating) is used to separate an optical signal into spatially separated wavelength channels incident onto a liquid crystal spatial light modulator (LC SLM). The SLM is then configured to rotate the polarisation of the light of a desired wavelength channel by 90° which causes the light to be deflected from the main channel by a birefringent crystal. The wavelengths are then recombined by a second grating element forming two spatially-displaced outputs: one containing the wavelength channels acted on by the LC SLM, and the second output containing the remaining wavelength channels. Since these types of switches are limited to only two polarisation states, they are not readily scalable, though more complicated schemes can be envisaged to allow for switching between multiple ports. With polarization switching, also, dynamic equalization of channels can only be done at the expense of the rejected light being channelled into the second fibre—so it is not applicable to equalization of the express path whilst dropping a number of wavelengths.
A better alternative to switch between multiple ports has been the use of optical beam deflectors such as MEMS mirror arrays or LC SLMs. These devices deflect the light through free space, thus allowing multiple signal beams to be simultaneously interconnected without cross-talk between data channels.
An example of a MEMS-based device is taught by Waverka (U.S. Pat. No. 6,501,877) which disperses the individual wavelength channels with a diffraction grating. The individual channels are each then focused on to spatially separated elements of the MEMS array which imparts an angular displacement on the beams. A retroreflection device is used to convert the angular displacement to a lateral offset, that when passed back through the optical system translates into a coupling to the desired output port. In this implementation the offset states are quantised and determined by the angles of the retroreflection prism.
A similar technique is taught in U.S. Pat. No. 6,707,959 by Ducellier where a particular spatially separated wavelength channel is acted upon by a deflector array implemented either using a MEMS device or a transmissive LC deflector. A schematic block diagram of this device is shown in FIG. 1. Ducellier introduces an improvement over Waverka by having the angle to offset (ATO) element 1 being able to translate continuously for an arbitrary state by placing an angle to offset lens at the Rayleigh point of the optical array 2. The angular array is then transmitted through a standard 4-f lens design (telecentric telescope) using a spherical reflector 3 to the deflection array 4 with preservation of the angular multiplex. The individual wavelength channels in the optical signal are separated by an optical dispersion element 5 at the telecentric point of the optical system.
The deflection array 4 can be operated in either reflective or transmissive mode and (similarly to Waverka and Patel) provides a deflection of a desired wavelength channel perpendicularly to the wavelength dispersion direction. The deflection is such that an ATO element at the output array translates the new angular multiplex into an offset corresponding to the desired output port. In this system, the input array, the optical dispersion element, the deflection array, and the output array all lie in the same focal plane due to the spherical symmetry of the optics. The disadvantage of this is that large deflection angles are required to switch between fibre ports and a requirement for large numerical aperture optics. The requirement also of a duplicate optical system in the transmissive deflection array embodiment places severe restrictions on the compactness and cost of the final device.
Additionally, none of the devices described above can operate on the light from two input sources or two groupings of light having the same wavelength channels independently. Due to the existence of polarisation dependent loss and polarisation mode dispersion—it is often convenient to consider two orthogonal polarisation states as two separate sources and it could be advantageous to act on these separately.
Various techniques have been proposed for the correction of polarisation dependent loss (PDL) in optical communication systems on a wavelength basis such as those discussed by Roberts (U.S. Patent Application Publication 2004/0004755). These techniques however are only applicable to a single optical fibre and operate in transmission mode only. To our knowledge, there have been no techniques have been proposed or demonstrated to provide broadband PDL correction for multiple optical fibre devices or in a switching architecture.
It is an object of the present invention to overcome or ameliorate at least some of the disadvantages of the prior art by providing a reconfigurable optical add/drop multiplexer and wavelength selective switch capable of independently operating on arbitrary wavelength channels contained in light from two distinct sources or groups.